Phase noise shaping in a distance measurement system

ABSTRACT

A distance measurement system includes a light transmitter to generate a modulated light signal, a light sensor to generate measurement signals from reflected light among four quad phase angles with respect to a phase of the generated light signal, and a controller. The controller selects a first set of quad phase angles, and generates first measurement signals at the quad phase angles of the first set. Based on the first measurement signals, the controller computes a first phase angle between the generated light signal and the reflected light signal, generates a second set of quad phase angles based on the first phase angle, and generates second measurement signals at the quad phase angles of the second set. Further, based on the second measurement signals, the controller computes a second phase angle between the generated light signal and the reflected light signal and calculates a distance using the second phase angle.

BACKGROUND

Three-dimensional (3D) time-of-flight (ToF) camera systems work bytransmitting light with periodically varying intensity and measuring thephase of the reflected light detected by photo sensors. The amount ofphase delay between the transmitted and reflected light signals isproportional to the distance between the camera system and the 3-Dobject. Therefore, distance is calculated form the measured phase delay.Various sources of noise are present in the measurements made by a 3DToF camera system. Examples of source of noise include photon shotnoise, pixel thermal noise (kTC), noise in the analog-to-digitalconverter (ADC) which produces a digital value from the measurements,and ADC quantization noise. Noise in the measurements causesinaccuracies in the computed distances.

SUMMARY

In accordance with at least one embodiment of the invention, a distancemeasurement system includes a light transmitter to generate a modulatedlight signal, a light sensor to generate measurement signals fromreflected light among multiple (e.g., four) quad phase angles withrespect to a phase of the generated light signal, and a controller. Thecontroller selects a first set of quad phase angles, and generates firstmeasurement signals at the quad phase angles of the first set. Based onthe first measurement signals, the controller computes a first phaseangle between the generated light signal and the reflected light signal,generates a second set of quad phase angles based on the first phaseangle, and generates second measurement signals at the quad phase anglesof the second set. Further, based on the second measurement signals, thecontroller computes a second phase angle between the generated lightsignal and the reflected light signal and calculates a distance usingthe second phase angle.

In another embodiment, a system includes a light transmitter configuredto generate a modulated light signal and a light sensor configured toreceive a reflected light signal and to generate measurement signalsamong four quad phase angles with respect to a phase of the generatedlight signal. The system also includes a controller coupled to the lightsensor. The controller is configured to dynamically vary the quad phaseangles and to calculate distance using the dynamically varying quadphase angles.

In yet another embodiment, a method includes generating a modulatedlight signal and selecting a first set of quad phase angles. Based areflected light signal, the method includes generating first measurementsignals at the quad phase angles of the first set. Further, based on thefirst measurement signals, the method includes computing a first phaseangle between the generated light signal and the reflected light signal.In addition, the method includes generating a second set of quad phaseangles based on the first phase angle, generating second measurementsignals at the quad phase angles of the second set, and based on thesecond measurement signals, computing a second phase angle between thegenerated light signal and the reflected light signal. A distance usingthe second phase angle is then calculated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 illustrates a distance measurement system in accordance withvarious examples;

FIG. 2 shows a set of waveforms illustrating operation of the distancemeasurement system in accordance with various examples;

FIG. 3 illustrates the relationship between a computed phase anglebetween transmitted and reflected light signals and the measurementsusing a set of quad phase angles in accordance with one example;

FIG. 4 shows another example of the relationship between a computedphase angle between transmitted and reflected light signals and themeasurements using a different set of quad phase angles;

FIG. 5 shows another set of waveforms illustrating operation of thedistance measurement system in accordance with various examples;

FIG. 6 includes a flowchart illustrating a method in accordance with anexample;

FIG. 7 shows an example of a use of the distance measurement system; and

FIG. 8 shows an example including a time delay in the transmit signalpath.

DETAILED DESCRIPTION

In accordance with the disclosed embodiments, a distance measurementsystem (e.g., a 3D ToF camera) transmits a modulated light signal, andcalculates the phase angle between the transmitted modulated lightsignal and the reflected light signal received at a receiver of thesystem. The calculation of the phase angle uses a set of time windowsthat are phase delayed relative to the phase of the transmittedmodulated light signal. In the illustrative example herein, there arefour time windows (referred to as “quads”) and each time window has adifferent phase delay relative to the transmitted modulated lightsignal. One illustrative set of quad phase angles includes 0 degrees, 90degrees, 180 degrees, and 270 degrees.

During each of this phase delayed time windows, the distance measurementsystem takes a measurement indicative of the amount of reflected lightreceived by the system during the respective time window. In oneexample, the reflected light signal impinges on a photo detector whichthen generates an electrical current. The generated electrical currentthen may be multiplied by a reference electrical signal (which may havethe same frequency as the light signal) and the multiplied resultingsignal is then used to charge a capacitor. The capacitor's voltage isrelated to the amount of reflected light received during the timewindow. Other techniques are possible as well for making the measurementsuch as digitizing the current and then integrating the current in thedigital domain. The time windows may be implemented by, for example,transistor switches that cause the photo detector to be coupled to ordecoupled from the circuit that makes the measurement. The timing (startand stop times) of time windows are controlled by a controller withinthe distance measurement device.

As mentioned above, various sources of noise such as photon shot noise,pixel thermal noise, noise in the analog-to-digital converter (ADC)which produces a digital value from the measurements, and ADCquantization noise can infect the measurements. In accordance with thedisclosed embodiments, the distance measurement system dynamicallyadjusts the quad phase angles to reduce the detrimental effects of suchnoise. Thus, during operation, the distance measurement system maychange the quad phase angles from a first set of quad phase angles(e.g., 0 degrees, 90 degrees, 180 degrees, and 270 degrees) to a secondset of quad phase angles that may include, for example, 0 degrees, 180degrees, and two other angles different than 90 degrees and 270 degrees.The calculation of the new quad phase angles is based on a determinationof the phase angle between the transmitted and reflected light signals,and example of the calculation is provided below. Once the new set ofquad phase angles is determined, the new set of quad phase angles areused to compute a second phase angle between the transmitted andreflected light signals. The resulting new set of quad phase angles issuch that the effects of noise on the second phase angle are less thanwould have been the case if the former set of quad angles were used.

FIG. 1 shows an example of a distance measurement system 100 usable tomeasure distance between the distance measurement system 100 and a 3Dobject 130. In one example, the distance measurement system 100 is a 3DToF camera. The distance measurement system 100 in this example includesa light transmitter 102, a modulator 104, optic lenses 106 and 110, areceiver array 108, an analog-to-digital converter (ADC) 112, and acontroller 114. Alternative or additional components may be included aswell. The transmitter 102 may include a solid-state laser or a lightemitting diode (LED) operating at, for example, a near-infraredwavelength such as 850 nm, although different wavelengths are possibleas well. The modulator 104 generates a modulation signal which is usedby the transmitter to generate a modulated light signal to betransmitted through lens 106. The modulation frequency of thetransmitted light 120 sets the maximum distance that can be measured bythe distance measurement system. As such, different modulation frequencycan be selected for different distance applications.

Transmitted light 120 reflects off a surface of the 3D object 130 andreflected light 125 is received through lens 110 into the receiver array108. The receiver array 108 may comprise multiple photo detectors 109.Each photo detector 109 generates a current based on the light receivedby the photo detector as well as a reference electrical signal 117 fromthe modulator 104 (as explained above), and the current can be used tomake a measurement as noted above. For example, the current can be usedto charge a capacitor whose resulting voltage is the measurement valueused to compute distance. The various photo detectors 109 comprising thereceiver array 108 may be used to compute distances to various points onthe surface of the 3D object 130 thereby creating, for example, a 3Dcontour map of the object.

The receiver array 108 is coupled to the ADC 112. Measurement signals111 from the receiver array 108 are digitized by the ADC 112 and digitalvalues 113 are provided to the controller 114 for further processing.The controller 114 may perform one or more of the operations describedherein upon execution of machine instructions (e.g., firmware). In otherembodiments, the controller 114 may comprise a programmable logicdevice, a discrete circuit or other type of circuit or device that canperform the operations described herein.

The controller 114 exposes the receiver array in distinct time windows(quads). Each quad has a different phase angle and the phase angle isthe phase difference between the transmitted light signal and thereference electrical signal. The controller 114 computes the quad phaseangles 115 and provides them to the receiver array 108 forimplementation. The quad phase angles may be implemented, for example,as a start timing signal relative to an edge of the transmitted lightsignal 120. The length of a given time window may be specified by thecontroller 114 as well as a time value relative to the start timingsignal.

FIG. 2 shows an example set of waveforms. The transmitted light signal120 is a modulated signal illustrated as a generally square wave withrising edges 121 and corresponding falling edges 122. The reflectedlight signal 125 is modulated the same as the transmitted light signal120 but is phase delayed by an angle θ from the transmitted light signal120. The edges 126 (and 127) of the reflected signal 125 correspond toedges 121 (and 122) of the transmitted light signal 120 but delayed dueto the distance over which the light travels. The amount of the phasedelay θ is a function of the distance between the distance measurementsystem 100 and the point on the surface of the 3D object 130 whichreflects the transmitted light signal 120.

FIG. 2 also illustrates a set of time windows C1, C2, C3, and C4corresponding to a set quad phase angles 0 degrees, 90 degrees, 180degrees, and 270 degrees. Time window C1 (0 degrees) is aligned to thephase of the transmitted light signal 120 whereas time window C2 (180degrees) is 180 degrees out of phase with respect to the transmittedlight signal as shown. Time windows C3 and C4 represent 90 degree and270 degree phase shifts from the transmitted light signal 120. Thecross-hatching shown for the various time windows represents the timeperiod during each such time window that the reflected light signal isimpinging on the photo detector. The time periods in which the reflectedlight signal impinges on the photo detector during the various timewindows is a function of the corresponding quad phase angle. Themeasurement signals recorded by the receiver array for the quad phaseangles thus may vary from time window to time window and are used by thecontroller 114 to calculate the phase angle θ between the transmittedand reflected light signals 120 and 125.

Equation (1) below represents an example of the relationship between thevarious quad phase angles and the measurements from the receiver array108.

$\begin{matrix}{{\begin{bmatrix}{\cos(0)} & {\sin(0)} \\{\cos\left( {\pi/2} \right)} & {\sin\left( {\pi/2} \right)} \\{\cos(\pi)} & {\sin(\pi)} \\{\cos\left( {3{\pi/2}} \right)} & {\sin\left( {3{\pi/2}} \right)}\end{bmatrix}\begin{bmatrix}I \\Q\end{bmatrix}} = \begin{bmatrix}{S\; 1} \\{S\; 2} \\{S\; 3} \\{S\; 4}\end{bmatrix}} & (1)\end{matrix}$where S1-S4 represent the measurements (e.g., voltage measurements)during the four time windows corresponding to the four quad phaseangles. Equation (1) is valid in the example in which transmitted andreflected modulated light signals are sinusoids and the photo detectorcurrent is a multiplication of the received light signal with theelectrical reference signal. The arguments of the cosine and sine valuesrepresent the four quad phase angles in radians. Thus, quad phase angles0 rad, π/2 rad, π rad, and 3π/2 rad corresponding to the angles 0degrees, 90 degrees, 180 degrees and 270 degrees, respectively. Equation(1) can be solved by the controller 114 for I and Q, which can then beused to calculate the phase angle θ between the transmitted andreflected light signals 120 and 125:

$\begin{matrix}{\theta = {\arctan\left( \frac{Q}{I} \right)}} & (2)\end{matrix}$The distance between the distance measurement system 100 and the 3Dobject then can be calculated as:

$\begin{matrix}{d = {\theta\left( \frac{c}{4{\pi f}} \right)}} & (3)\end{matrix}$where c is the speed of light and f is the modulation frequency.

FIG. 3 illustrates vectors corresponding to each of the four measurementvalues S1-S4. An illustrative calculated phase angle θ also is shown.The magnitude of the vector 140 defining the phase angle θ is thecalculated distance d. The dashed region 150 represents the uncertaintyin the calculation of the phase angle θ and thus the distance d. In thisexample, the noise region 150 is approximately circular and the actualdistance d may vary from that shown bounded by the noise region 150.

In accordance with illustrative embodiments, the controller 114 maymodify the quad phase angles to be used to make the various measurementsS1-S4 to thereby change the shape of the noise region. The shape can bechanged to compress it in the direction of the distance vector. FIG. 4,for example, illustrates an example in which the set of quad phaseangles include 0 rad, 3π/4 rad, π rad, and 7π/4 rad. The 0 rad and π radquad phase angles are retained but the π/2 rad and 3π/2 quad phaseangles have been changed to 3π/4 rad and 7π/4 rad. The 3π/4 rad (S2) and7π/4 rad (S4) quad phase angles have been calculated by the controller114 to define an axis 160 such that the vector 140 defined by angle θbisects the angle defined by S1 and S2, and thus θ equals α1 in FIG. 4.With the quad phase angles recalculated in this manner for phase angleθ, a new phase angle θ′ is calculated. The shape of the noise region 155for the new phase angle θ′ is compressed for the range of possible phaseangle calculations as shown thereby reducing the uncertainty range ofthe phase angle. With a tighter possible range of possible phase angles,the calculated distance d′ advantageously has a narrower range as well.

In another example, the new set of quad phase angles can all bedifferent than the former set of quad phase angles. In one such example,the new set of quad phase angles are calculated by the controller 114such that the vector defined by phase angle θ is perpendicular to theaverage of the set of quad phase angles. That is,

$\begin{matrix}{\theta = {{\frac{1}{n}*{\sum\limits_{i = 1}^{n}\alpha_{i}}} \pm {\pi/2}}} & (4)\end{matrix}$where n represents the number of quads (4 in the disclosed examples). Insome embodiments, the controller 114 determines the quads to satisfyequation (4) and to minimize

${{\frac{1}{n}*{\sum\limits_{i = 1}^{n}\alpha_{i}}} \pm \frac{\pi}{2}} - {\theta.}$

The generalized form of equation (1) is given as follows:

$\begin{matrix}{{\begin{bmatrix}{\cos({\varphi 1})} & {\sin({\varphi 1})} \\{\cos({\varphi 2})} & {\sin({\varphi 2})} \\{\cos({\varphi 3})} & {\sin({\varphi 3})} \\{\cos({\varphi 4})} & {\sin({\varphi 4})}\end{bmatrix}\begin{bmatrix}I \\Q\end{bmatrix}} = \begin{bmatrix}{S\; 1} \\{S\; 2} \\{S\; 3} \\{S\; 4}\end{bmatrix}} & (5)\end{matrix}$where φ1 through φ4 represent the four quad phase angles.

FIG. 5 shows the waveforms of FIG. 2 but with the time windows C3 and C4recalculated to have different quad phase angles (120 degrees which is3π/4, and 300 degrees which is 7π/4). As a result, the amount (timeexposure) of reflected light received by the receiver array during theC3 time window is increased relative to that of FIG. 2.

FIG. 6 illustrates a method in accordance with various embodiments. Theoperations may be performed in the order shown, or in a different order.Further, the operations may be performed sequentially, or the two ormore of the operations may be performed concurrently. The operations maybe performed by, or under the control of, the controller 114 of FIG. 1.

At 208, the method includes transmitting a modulated light signal with,for example, an average modulation frequency f. The modulated lightsignal may be continuously transmitted as soon as the distancemeasurement system 100 is turned on, or may be transmitted upon a useractivating a control (e.g., a button) on the system. The transmittedlight signal may be in the near-infrared part of the electromagneticspectrum, although other wavelengths may be implemented as well for thetransmitter.

At 210, the method includes selecting a first set of quad phase angles.In one example, the first set of quad phase angles includes 0 degrees,180 degrees, 90 degrees and 270 degrees, although a different set ofquad phase angles may be selected. At 212, the method includesgenerating a first set of measurement signals at the quad angles of thefirst set. For example, the voltage on a capacitor may be monitoredduring each of the time windows corresponding to each of the quad phaseangles of the first set.

At 214, based on the first set of measurement signals, the methodincludes computing a first phase angle between the transmitted lightsignal and the reflected light signal. In one embodiment, thiscomputation may comprise solving equation (5) above for I and Q and thencomputing the phase angle using equation (2). The phase angle calculatedin operation 214 is an estimate of the phase angle between thetransmitted and reflected light signals.

At 216, the illustrative method includes generating a second set of quadphase angles based on the phase angle calculated in 214. An example ofthis operation is to retain the quad phase angles 0 and 180 degrees (0and π rad) and compute an additional two quad phase angles as explainedabove. If the phase angle computed at 214 is less than the second lowestquad phase angle (e.g., less than 90 degrees if 0, 90, 180, and 270degree quad phase angles are used), then one quad phase angle defines anaxis that bisects 0 and the second lowest quad phase angle. However, ifthe phase angle computed at 214 is greater than the second lowest quadphase angle (e.g., greater than 90 degrees if 0, 90, 180, and 270 degreequad phase angles are used), then one quad phase angle defines an axisthat bisects axis defined by the second lowest quad phase angle and 180degrees. In either, case, a fourth quad phase angle is computed to besupplementary the newly calculated quad phase angle. Another example isprovided above in which a second set of quad phase angles is computed sothat the phase angle θ is perpendicular to the average of the set ofquad phase angles (see eq. (4) above).

At 218, the method includes generating second measurement signals at thequad phase angles of the second set. This operation is similar to thatof operation 212 but with a set of quad phase angles that is differentthan that used in operation 212. At 214, based on the second set ofmeasurement signals, the method includes computing a second phase anglebetween the transmitted light signal and the reflected light signal. Asexplained above, this computation may comprise solving equation (4)above for I and Q and then computing the phase angle using equation (2).At 222, the method includes calculating a distance using the secondphase angle.

As explained above, distance can be calculated from a calculation of thephase angle θ between the transmitted and reflected light signals. Thesize of the phase angle θ is a function of the time it takes for thelight to travel to, and reflect off of, the 3D object and be received bythe receiver array 108. Thus, the phase angle θ will be different forlight that is reflected off different surface points of the 3D object.Surface points that are farther from the distance measurement system 100will have larger phase delays than for surface points that are closer.In general, the signal-to-noise ratio (SNR) is smaller for more distantpoints than for closer points. Accordingly, in some embodiments, thecontroller 114 may dynamically compute quad phase angles using thelargest phase angle θ computed from the receiver array 108. That is, thecontroller calculates the phase angles θ for each of the photo detectors109 in the receiver array and thus to various points on a 3D object orscene. The largest calculated phase angle corresponds to the mostdistant point and the quad phase angles are dynamically determined forthe subsequent assessment of distances using that particular phaseangle.

One application of the distance measurement system 100 is to compute a3D distance map to a time varying scene (e.g., a live scene for whichdistances to the various surfaces in the scene vary with respect to thedistance measurement system). The distance measurement system 100 maycalculate a set of distances for each of the photo detectors 109 of thereceiver array 108 at various time intervals (e.g., periodic timeintervals) such as 30 times per second (30 “frames” per second). Thequad phase angles thus can be adjusted between successive frames. For agiven frame, the phase angles are computed for the various photodetectors and for the next frame, the quad phase angles are adjustedbased on the largest phase angle computed from the previous frame.

FIG. 7 illustrates another use of the distance measurement system 100and how the quad phase angles can be dynamically changed. In thisexample, a 3D scene 300 includes a person holding up his hand. As such,the person's hand is closer to the distance measurement system 100 thanthe person's head and the rest of the background. In some embodiments, aportion of a scene may be of greater interest for distance tracking thanother portions of the scene. In the example of FIG. 7, the person's handis of greater interest. In such embodiments, the controller 114 maycompute the quad angles based on the calculated phase angle θ to thescene portion of interest. A bounding box 310 is shown encompassing theperson's hand (and the portion of interest in general). The system 100could decide the portion of interest differently depending on theapplication for which it is being used. For obstacle avoidance, it wouldlook at a rectangle of a fixed size with pixels which show the smallest(nearest) phase value. If it is a gesture recognition system, it wouldlook for specific patterns like a hand, arm, etc. Such determinations ofthe portion of interest may be performed by a processor separate fromthe controller 114 of the distance measurement system 100.

The distance measurement system 100 calculates the phase angles θ fordifferent points within the bounding box 310, and then assigns eachcomputed phase angle to one of multiple bins. Each bin defines arelatively small range of phase angles (or a single phase angle). Byassigning each calculated phase angle to its corresponding bin, a countof the number of phase angles within each bin can be determined. Thecontroller 114 determines (312) the “mode” of the bounding box to be thephase angle θ of the bin with the largest number of phase angles. If thebin is mapped to a range of phase angles, the mode of the bin may becomputed as the average phase angle or may be selected to be one of thephase angles in the range. At 314, the controller calculates the quadphase angles based on the determined mode's phase angle θ.

Distance is calculated as explained herein based on phase angle, whichwraps around every 2π radians, resulting in an aliasing distance. Thedistance at which aliasing occurs is referred to as the ambiguitydistance, damb, and is inversely related to the modulation frequency, f,as:

$\begin{matrix}{{damb} = \frac{c}{2f}} & (6)\end{matrix}$

The ambiguity distance, damb, is the maximum measurable distance for agiven frequency. To increase the measurable distance, the frequency fcan be lowered but at the cost of reduced accuracy.

In accordance with the disclosed embodiments, multiple (e.g., 2)different frequencies are used concurrently. The light transmitter 102in FIG. 1 can transmit multiple light signals with different modulationfrequencies. Measuring the same object with two (or more) differentfrequencies produces two different phase angles between the transmittedand reflected light signals. The combination of two phase angles isunambiguous for a distance that is longer than for either of thefrequencies individually. For example, the unambiguous distance for eachof the frequencies 60 MHz and 80 MHz is 2.498 m and 1.875 m,respectively. However, concurrently using both 60 MHz and 80 MHztogether results in an unambiguous distance of 7.5 m. In suchembodiments, the system 100 performs measurements with both frequenciesconcurrently.

For the individual frequency measurements to be discernible, the cos/sinmatrix of equation (5) should be invertible. The modified version ofequation (5) to accommodate two different frequencies may be:

$\begin{matrix}{{\begin{bmatrix}{\cos({\varphi 1})} & {\sin({\varphi 1})} & {\cos({\theta 1})} & {\sin({\theta 1})} \\{\cos({\varphi 2})} & {\sin({\varphi 2})} & {\cos({\theta 2})} & {\sin({\theta 2})} \\{\cos({\varphi 3})} & {\sin({\varphi 3})} & {\cos({\theta 3})} & {\sin({\theta 3})} \\{\cos({\varphi 4})} & {\sin({\varphi 4})} & {\cos({\theta 4})} & {\sin({\theta 4})}\end{bmatrix}\begin{bmatrix}{I\; 1} \\{Q\; 1} \\{I\; 2} \\{Q\; 2}\end{bmatrix}} = \begin{bmatrix}{S\; 1} \\{S\; 2} \\{S\; 3} \\{S\; 4}\end{bmatrix}} & (7)\end{matrix}$I1 and Q1 are the I,Q components of the first frequency's vector. I2 andQ2 are the I,Q components of the second frequency's vector. ϕn (n is 1,2, 3, 4) are the quad phase angles of the first frequency, and en arethe quad phase angles for the second frequency. Each frequency's quadphase angles can be optimized to obtain the noise shaping advantage asis described above. Although two different frequencies are noted above,the technique can be extended to any number of frequencies.

FIG. 8 illustrates an embodiment similar to that of FIG. 1, but one thatincludes a controllable time delay 121 in the transmitted light signalpath). The time delay 121 may be a a hardware time delay circuit or asoftware-controlled time delay. The amount of the time delay implementedby the time delay 121 can provide additional control over the timeamount of time delay between the transmitted light signal and the quadtime windows.

In the waveform examples of FIGS. 2 and 5, the length of each timewindow C1-C4 (designated in FIG. 2 as T1) represents a portion of theintegration time for the measurements S1-S4. In the examples of FIGS. 2and 5, the integration times for the various quad phase angles may belonger than T1 (e.g., a few orders of magnitude longer than T1) and maybe the same. However, in other embodiments the integration time (or T1)of one quad phase angle may be different than the integration time (T1)of another quad phase angle. The integration times of the various quadphase angles is controlled by controller 114 and can be varied tofurther shape the noise region of each calculated phase angle θ. Thedirection in which there is more integration time will have a largerSNR, and the direction with less integration time will have a smallerSNR. One illustrative strategy can be for the controller 114 to assignmore integration time to directions perpendicular to the expected vectordefined by the angle θ and distance d. This is an alternative to varyingthe amount of noise in a given direction. This latter technique may notrotate the noise envelope, but it can change the shape of the noiseenvelope. So when the quad angles cannot be set to values accurateenough (e.g., due to hardware limitations), the controller 114 insteadcan control the integration time for quads in each direction.

Certain terms are used throughout the following description and claimsto refer to particular system components. Different companies may referto a component by different names. This document does not intend todistinguish between components that differ in name but not function. Inthe following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” Also, theterm “couple” or “couples” is intended to mean either an indirect ordirect wired or wireless connection. Thus, if a first device couples toa second device, that connection may be through a direct connection orthrough an indirect connection via other devices and connections. Theabove discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A distance measurement system, comprising: alight transmitter configured to generate a modulated light signal; alight sensor configured to receive a reflected light signal and togenerate measurement signals among four quad phase angles with respectto a phase of the generated light signal; and a controller coupled tothe light sensor and configured to: select a first set of quad phaseangles; generate first measurement signals at the quad phase angles ofthe first set; based on the first measurement signals, compute a firstphase angle between the generated light signal and the reflected lightsignal; generate a second set of quad phase angles based on the firstphase angle; generate second measurement signals at the quad phaseangles of the second set; based on the second measurement signals,compute a second phase angle between the generated light signal and thereflected light signal; and calculate a distance using the second phaseangle; wherein: the first set of quad phase angles comprises 0 degrees,90 degrees, 180 degrees and 270 degrees; the second set of quad phaseangles includes 0 degrees and 180 degrees; the controller is configuredto compute a third quad phase angle such that the first phase angledefines a vector that bisects the third quad phase angle; and thecontroller is configured to compute a fourth quad phase angle that issupplementary to the third quad phase angle.
 2. The distance measurementsystem of claim 1, wherein the controller is further configured to:generate the first measurement signals at the quad phase angles of thefirst set for each of multiple points of a three-dimensional scene;based on the first measurement signals for each of the multiple points,compute a first phase angle between the generated light signal and thereflected light signal for each of the multiple points; compute adistance to each of the multiple points based on the first phase anglesfor the multiple points; and for a particular point of the multiplepoints whose computed distance is longer than the distances computed forother of the points, generate the second set of quad phase angles basedon the first phase angle computed for that particular point.
 3. Thedistance measurement system of claim 2, wherein the controller isfurther configured to calculate, for each point, the second phase anglebetween the generated light signal and the reflected light signal, andto calculate a distance to each of the multiple points using thecorresponding second phase angles.
 4. The distance measurement system ofclaim 1, wherein the controller is configured to define an integrationtime for each quad phase angle for generation of the measurementsignals, and wherein the integration time for at least one quad phaseangle is different than the integration time for at least one other quadphase angle.
 5. The distance measurement system of claim 1, wherein thelight transmitter is configured to generate multiple light signals atdifferent modulation frequencies, and wherein the controller isconfigured to calculate the distance using the multiple light signals.6. The distance measurement system of claim 5, wherein the controller isconfigured to compute separate first phase angle between the generatedlight signal and the reflected light signals for each of the multiplefrequencies.
 7. A method, comprising: generating a first modulated lightsignal; selecting a first set of quad phase angles; based a reflectedlight signal, generating first measurement signals at the quad phaseangles of the first set; based on the first measurement signals,computing a first phase angle between the generated light signal and thereflected light signal; generating a second set of quad phase anglesbased on the first phase angle; generating second measurement signals atthe quad phase angles of the second set; based on the second measurementsignals, computing a second phase angle between the generated firstmodulated light signal and the reflected light signal; and calculating adistance using the second phase angle; wherein: generating the secondset of quad phase angles includes computing a quad phase angle such thatthe first phase angle defines a vector that bisects the third quad phaseangle.
 8. The method of claim 7, further comprising defining anintegration time for each quad phase angle for generating themeasurement signals, and wherein the integration time for at least onequad phase angle is different than the integration time for at least oneother quad phase angle.
 9. The method of claim 7, further comprisinggenerating a second modulated light signal with a different modulationfrequency than for the first modulated frequency, and wherein selectingthe first set of quad phase angles comprises selecting a set of quadphase angles for each of the modulation frequencies, and whereincalculating the distance includes using reflected light signals withboth modulation frequencies.